Generally, an ultrasonic sensor can be provided to detect an obstacle positioned around a vehicle, for example. As disclosed in JP-2004-264264A, a hole is arranged at a bumper of the vehicle. The head portion of the ultrasonic sensor is exposed from the hole in such a manner that the surface of the head portion is in a same surface with an outer surface of the bumper. In this case, the head portion of the ultrasonic sensor is exposed outside the vehicle. It is not preferable in terms of design.
As disclosed in JP-10-123236A, a recess is provided at the back side of the bumper of the vehicle to accommodate therein an ultrasound vibration type sensor (ultrasonic sensor). In this case, the ultrasonic sensor becomes invisible from the exterior of the vehicle, thus maintaining an outward appearance of the vehicle.
With reference to JP-10-123236A, the ultrasonic sensor has an ultrasonic vibrator, which is constructed of a disk-shaped ceramic and integrally stacked at a metallic base. The ultrasonic sensor is arranged at the recess, with an outer surface of the ceramic directly contacting a bottom of the recess. In this case, it is not specifically taken into consideration to obtain a desired directivity.
However, in the case where the obstacle detection device is used for a bumper of a vehicle, it is generally demanded that the directivity of the obstacle detection device in a horizontal direction relative to the ground surface is wide and that in a vertical direction is narrow, in order to restrict an erroneous detection due to a road surface, an edge stone and the like.